Possibly the closest Super Bowl matchup in history also poses the question: how much does it mean when certain aspects of an NFL team improve dramatically in the second half of the season?
10 Sep 2008
by Brian Fremeau
The Fremeau Efficiency Index (FEI) principles and methodology can be found here. Like DVOA, FEI rewards playing well against good teams, win or lose, and punishes losing to poor teams more harshly than it rewards defeating poor teams. Unlike DVOA, it is drive-based, not play-by-play based, and it is specifically engineered to measure the college game.
FEI is the opponent-adjusted value of Game Efficiency (GE), a measurement of the success rate of a team scoring and preventing opponent scoring throughout the non-garbage-time possessions of a game. Like DVOA, it represents a team's efficiency value over average.
Only games between FBS teams are considered. Since limited data is available at the beginning of the season, the ratings to date are a function of both actual games played and projected outcomes based on the 2008 Projected FEI Ratings. The weight given to projected outcomes will be reduced each week until mid-October, at which point the projections will be eliminated entirely.
It is still too early in the season to make absolutely conclusive statements about teams. But even though the projected results still carry significant weight, a couple of things stand out in the ratings. A total of three non-BCS conference teams appear in the FEI top 25 (BYU, Utah, and East Carolina), one more than the combined total of the ACC (two) and Big East (zero). Virginia Tech will drift down in the coming weeks if they don't meet their projected expectations, and the Hokies are only hanging in the top 10 now on the strength of those projections and a close loss to what looks to be a very good East Carolina team. No. 12 Florida State won't face its first FBS opponent for ten more days, so it looks like they're safe for now.
I still expect several teams in these "power" conferences to recover some dignity from early season letdowns, but the cumulative effect of multiple-score losses to South Carolina (by North Carolina State), Alabama (Clemson), Bowling Green (Pittsburgh), Northwestern and Akron (Syracuse), USC (Virginia), Fresno State (Rutgers), East Carolina (West Virginia), Florida (Miami), Middle Tennessee (Maryland) and Oklahoma (Cincinnati) will never be fully overcome. The ACC and Big East in total have recorded a grand total of two wins against out-of-conference BCS competition, including a 41-13 victory by Wake Forest over lowly Baylor that barely qualifies. Forget the impending BCS controversy if both conference champions finish behind multiple non-BCS league teams. I want to know if these conferences will be able to qualify 14 teams to fill out their bowl bids at the end of the season.
When FEI was first introduced two years ago, the column included a single paragraph on Home Field Advantage, largely dismissing it as statistically insignificant. Believe it or not, this frustrated me as much as it frustrated some FO readers. HFA is supposed to be one of the eternal truths in sports, perhaps even in life. Death, Taxes, Home Field Advantage. Playoff systems in sports other than college football are deliberately structured to reward high-performing teams with the advantages of playing at home. Fans create hostile environments for visiting opponents and motivate the home team with feverish support. The negative effects of travel and foul weather, the potential for unfavorable officiating, and the unfamiliarity of the other guys' turf are all supposed to negatively impact the visitors. So why did FEI dismiss it then, and better yet, what's happening going forward?
The question for me was never about whether HFA could or could not be observed. The question for me was how precisely it could be measured and how it could be used. Commonly, HFA is understood to be worth three points per game in the NFL, and somewhere between three and five points in college football. Three is a quick-and-dirty number that's easily agreeable for both the casual and educated fans, but what is it supposed to mean in practical terms? Are teams, on average, supposed to make an extra field goal at home that they would have missed at a neutral site? Is it more accurate to name it Road Field Disadvantage, and punish the visitor rather than reward the host team? Does that distinction really matter? And biggest question for me: Does mis-matched opposition skew measurements of how much college football HFA is really worth? Can we calibrate it somehow?
Two weeks ago, I introduced Projected Win Expectation as a method to project college football team records from FEI data. As it turns out, there is a very strong relationship between PWE and Power Advantage, the difference in standard deviation of two teams' FEI ratings. Figure 1 plots the PWE/PA relationship for all teams in blue and home teams in red.
|Projected Win Expectation vs. FEI Power Advantage|
Teams clearly increase their likelihood of winning by playing at home, particularly in games between teams of relatively equal power. Games that would be 50/50 on a neutral field are won 60 percent of the time by the home team. PWE is boosted by at least five percent for home teams in more than two-thirds of all college football games every year. But as the power advantage grows, of course, the HFA effect on PWE becomes negligible and irrelevant. A team that is likely to dominate its overmatched foe and win 98 percent of the time on a neutral field can't elevate that likelihood much more by playing at home.
Figure 2 represents the full Home Team PWE Advantage distribution for both team Power Advantages and Disadvantages. Note the imbalance in the graph around the zero PA axis. With the exception of significant mismatches, playing at home is markedly more valuable to teams with a Power Disadvantage than it is to home teams with a reciprocal Power Advantage. In other words, if the No. 10 and No. 25 teams play, the PWE Advantage would be greater if the game were at the lower ranked team's stadium than if it were at the higher one. Home underdogs are popular picks for upset specials, and the distribution in Figure 2 helps illustrate why.
|Home Team PWE Advantage vs. FEI Power Advantage|
Interestingly, the effect of HFA on Game Efficiency, the foundation of FEI, is not at all dependent on Power Advantage or Disadvantage. Home teams play on average about four percent more efficiently than their road opposition, and that average holds up for large PA games as well as evenly-matched ones. Converting GE into points, however, is dependent on another variable: the number of possessions in the game. Solving the Game Efficiency formula for a national average number of game possessions, 24, and HFA in terms of points calculates to 3.4 points per game. As the number of competitive possessions in the game increases or decreases, the value of HFA proportionately follows suit at just under 0.5 points per three possessions.
Pace and style of play are big factors in determining the number of competitive possessions in a given game, but teams don't categorically play a constant possession-per-game rate. Nor do they fit a certain style. Florida and Navy averaged the fewest competitive possessions per game last season (21 per game), and both had very efficient, though very different offenses. Navy's ball control running attack generally reduced overall game possessions; Florida frequently raced out to fast leads, triggering garbage time earlier in the game.
In summary, Home Field Advantage is clearly measurable, but it can be calculated and used by FEI in different ways. Beginning with this week's ratings, the weekly FEI adjusts Game Efficiency data to compensate for the four percent home team GE boost and the weekly FEI Forecasts for all games published here include Projected Win Expectations adjusted according to the Figure 2 distribution. Home Field Advantage is still somewhat intangible, and further research could illuminate other measurements of team-specific HFA, weather-specific HFA, and more. Any and all suggestions are welcome.
16 comments, Last at 12 Sep 2008, 9:58am by Ross